[Full oral argument audio and transcript here.]
It's always risky trying to predict how the Court will decide just based on oral arguments. One reason is some Justices like Kennedy and Roberts are very skillful at asking tough questions of both sides. If I had to hazard a guess as to how the case will be decided however, I'd have to guess that it would be a narrow decision that goes nowhere near saying gay marriage should be legal across the land.
There are two main reasons why I believe the decision will be narrow. First, is several justices expressed concern about the "newness" of gay marriage. The Supreme Court, an institution with no enforcement power, damages its own credibility when it makes sweeping decisions that go ignored because the President will not enforce it. The Court often talks about the "ripeness" of an issue just like we talk about the ripeness of fruit. By "ripe" the Court means that the nation has come to a broad and enduring consensus about a social issue. While the momentum is shifting in favor of gay marriage, there is still evidence that the shift it not decisive yet . As a branch of government charged with being undemocratic (the Justices being unelected and unfireable by the other branches), the Court is very wary of stepping to far out from public opinion.
Alito in particular spoke about the "need to be cautious" because "same sex marriage is very new...newer than cell phones or the internet." But Justice Sotomayor also asked about why the Court is taking the case now. She noted, that the Court usually lets issues "perk" (percolate) and that the Court let racial segregation perk 50 yrs.
Second, there was a significant bit of discussion about "standing", or whether one of the parties could legally bring the suit anyway. Kennedy wondered out loud whether the case was granted certiorari in error, meaning that the Supreme Court should not have taken the case. Under federal law, one must show how one would be harmed by the legislation in order to sue. Clearly the gay couples have standing, but the other party must also demonstrate standing to bring the suit.
The fact that Kennedy was speculating about standing combined with his remarks a few weeks ago about his discomfort of 9 insular men and women from a "narrow background" deciding hot button issues for the nation, lead me to believe that Kennedy is 1) not among one of the four justices who voted to grant the case cert and 2) he is going through the technical issue of standing as a way of providing the Court with an out so that they don't have to decide.
If the Court ends up dismissing Hollingsworth v Perry based on standing, then it also invalidates the Ninth Circuit's decision since the party presumably did not having standing in that case either. That would leave Judge Vaughn Walker's broad decision in favor of gay marriage from the U.S. District Court intact. There are of course other possible legal outcomes and they are outlined here by Lyle Denniston at scotusblog.
Today's U.S. v Windsor about DOMA will also be instructive. More Tweet compilations and analysis here later. Oral argument expected to commence at 10am EST.